LEL and UEL
A flammable gas or vapor can only burn or explode when it's mixed with air within a certain range:
- The lower explosive limit (LEL), also called the lower flammable limit (LFL), is the lowest concentration in air that can ignite. Below it, the mixture is too lean.
- The upper explosive limit (UEL) is the highest concentration that can ignite. Above it, the mixture is too rich, but it becomes flammable again as air mixes in, so "too rich" is never safe.
Typical values
| Gas | LEL (% by volume) | UEL (% by volume) |
|---|---|---|
| Methane (natural gas) | 5.0% | 15% |
| Propane | 2.1% | 9.5% |
| Hydrogen | 4.0% | 75% |
| Hydrogen sulfide | 4.0% | 44% |
| Gasoline vapor | about 1.4% | about 7.6% |
Values vary slightly by source; check the substance's safety data sheet.
Reading %LEL on a monitor
A combustible gas sensor reports the concentration as a percentage of the LEL, not as a percentage of the air.
OSHA's 10 percent rule
Under 29 CFR 1910.146(b), flammable gas, vapor or mist in excess of 10% of its LFL is a hazardous atmosphere. That's why most monitors alarm at 10% LEL and most permits require readings below 10% LEL for entry. The margin allows for uneven mixing, pockets of gas and instrument tolerance. (In excavations, OSHA's limit is 20% of the LFL, under 1926.651(g)(1)(iii).)
Sensor limitations
- Calibration gas: catalytic sensors are usually calibrated to methane or pentane. Other gases read higher or lower; use the manufacturer's correction factors for the gas you expect.
- Oxygen needed: catalytic (pellistor) sensors need oxygen to work and can read falsely low in oxygen-deficient air, which is one reason oxygen is tested first.
- Poisons and inhibitors: silicones, lead compounds and some sulfur compounds, including H2S, can damage or reduce sensitivity of catalytic sensors.
- Infrared sensors don't need oxygen and resist poisoning, but don't detect hydrogen.
- Over-range: very high concentrations above the UEL can produce misleading readings. Treat an over-range alarm as a flammable atmosphere.
- Combustible dust isn't measured by gas sensors. OSHA's visual guide is dust that obscures vision at 5 feet or less.
Bump test daily and calibrate per the manufacturer. See bump test vs. calibration.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA. It does not replace the site-specific training, written program and hands-on evaluation your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.